High speed downlink packet access (HSDPA) is a feature that was introduced in Release 5 of the third generation partnership project (3GPP) specification. HSDPA achieves maximum spectral efficiency using three key concepts: adaptive modulation and coding (AMC), fast physical layer retransmissions implementing hybrid automatic repeat request (HARQ), and fast Node B scheduling.
Handover is a process in which a wireless transmit/receive unit (WTRU) switches from one cell to another without service interruption. In HSDPA, the WTRU monitors a high speed shared control channel (HS-SCCH) in a single cell, which is called a “serving high speed downlink shared channel (HS-DSCH) cell”. When a handover occurs, the WTRU needs to switch to a new serving HS-DSCH cell (target cell) and stop communication with the old serving HS-DSCH cell (source cell). This procedure is called serving HS-DSCH cell change.
Two types of handovers exist: synchronized and unsynchronized handovers. In an unsynchronized handover, the network and the WTRU do not activate the resources and switch at the same time. The activation time for the WTRU is set to “now.” This reduces the delays associated with the handover procedure, but it increases the probability of losing data.
In a synchronized handover, the network and the WTRU perform the change of resources simultaneously. The network has to set the activation time to a conservative value to account for any kind of delays such as scheduling delay, retransmissions, configuration time, etc. Even though the synchronized handover minimizes data losses it may result in higher delays.
Conventionally, the radio resource control (RRC) handover message is sent to the WTRU via the source cell. The delay associated with the serving HS-DSCH cell change may cause the handover message to fail, thus resulting in an unacceptable rate of dropped calls.
In order to improve the reliability of the serving HS-DSCH cell change procedure, it has been proposed to pre-configure target cell information or parameters. The pre-configuration of the target cell information or parameters adds robustness to the serving HS-DSCH cell change procedure by allowing the network to send the serving HS-DSCH cell change command either over the source cell or the target cell. The use of target cell pre-configuration is configured by the network during the active set update procedure.
A WTRU continuously measures the signal strength of neighboring cells. Based on the measurements on neighboring cells, a WTRU transmits a MEASUREMENT REPORT 1A or 1C message containing intra-frequency measurement results requesting addition of a new cell into the active set. As part of the active set update procedure, the SRNC establishes a new radio link in the target Node B for the dedicated physical channels. The SRNC then transmits an ACTIVE SET UPDATE message to the WTRU. The ACTIVE SET UPDATE message includes the necessary information for establishment of the dedicated physical channels in the added radio link. If the SRNC decides to pre-configure the target cell, the ACTIVE SET UPDATE message will also include serving HS-DSCH cell related configuration, (e.g., H-RNTI, HS-SCCH configuration, etc.), for the target cell. When the WTRU has added the new radio link, the WTRU returns an ACTIVE SET UPDATE COMPLETE message.
When the WTRU detects a change of best cell, the WTRU transmits a MEASUREMENT REPORT 1D message to request the change of the HS-DSCH serving cell to a target cell. The report contains the measured value and the cell identity (ID). The WTRU then starts monitoring one of the pre-configured HS-SCCH(s) in the target cell in addition to the HS-SCCH(s) in the source cell.
Upon reception of this event, a serving RNC (SRNC) makes a decision to perform a handover to a new cell. The SRNC requests the controlling RNC (CRNC) to allocate HS-DSCH resources, (such as HS-DSCH radio network temporary identity (H-RNTI), HS-SCCH codes, HARQ resources, etc.), for the WTRU in the target cell via radio network subsystem application part (RNSAP) and Node B application part (NBAP) messages. Once the HS-DSCH resources are reserved, the CRNC provides all the information to the SRNC, which in turn sends an RRC handover message to the WTRU over the source cell.
The SRNC may send a RADIO BEARER SETUP message, a RADIO BEARER RECONFIGURATION message, a TRANSPORT CHANNEL RECONFIGURATION message, or a PHYSICAL CHANNEL RECONFIGURATION message, which indicates the target HS-DSCH cell and optionally an activation time to the WTRU. The RRC message may also include a configuration of transport channel related parameters for the target HS-DSCH cell, including an indication to reset the MAC-hs or MAC-ehs entity.
In parallel, the target Node B may transmit an HS-SCCH order in the target cell to initiate the serving HS-DSCH cell change. This HS-SCCH order may be referred to as a HS-DSCH serving cell change order, or as target cell HS-SCCH order. If the WTRU has not received the RRC message, (i.e., RADIO BEARER SETUP message, RADIO BEARER RECONFIGURATION message, TRANSPORT CHANNEL RECONFIGURATION message, or PHYSICAL CHANNEL RECONFIGURATION message), the WTRU will upon receiving the HS-SCCH order in the target cell execute serving HS-DSCH cell change.
When the WTRU has completed the serving HS-DSCH cell change, the WTRU returns a RADIO BEARER SETUP COMPLETE message, a RADIO BEARER RECONFIGURATION COMPLETE message, a TRANSPORT CHANNEL RECONFIGURATION COMPLETE message, or a PHYSICAL CHANNEL RECONFIGURATION COMPLETE message to the network, regardless if the cell change was triggered by reception of the RRC message in the source cell or by reception of the HS-SCCH order in the target cell.
When an enhanced serving cell change procedure is used, the network will be configured to send the RRC message to the WTRU over the source cell or an HS-SCCH order over the target cell if configured. However, the RRC message might not be successfully delivered due to degrading radio conditions in the source cell. In addition, a problem occurs when the WTRU receives both RRC message and the target cell HS-SCCH order and they conflict with each other. For instance, when a target cell HS-SCCH order is received, the RRC entity in the WTRU is supposed to perform the serving HS-DSCH cell change within a required time period, (i.e., 40 ms). However the RRC message may contain a connection frame number (CFN) as an activation time that occurs much later than the 40 ms limit. Additionally, the RRC message may contain new configuration parameters that may conflict with the pre-configured HS-DSCH parameters. This may cause ambiguity in the WTRU. On the network side, this may cause problems since at the time the network considers the WTRU is ready and rerouted data to the target cell, the network may not know which configuration the WTRU is using and at what time the reconfiguration is performed, (i.e., 40 ms after the target cell HS-SCCH order or at the activation time given in the RRC message). This is due to the fact that the WTRU might have not received the RRC message and may have reconfigured using the old pre-configured parameters. In addition, in case both messages are received, the WTRU behavior on how and which message to process in the RRC entity have to be defined.